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Prenatal blood test for Down syndrome shows high level of accuracy

Prenatal testing to detect Down syndrome is carried out with amniocentesis or chorionic villus sampling. Both are invasive tests that carry about a 1% risk of miscarriage. Researchers said Tuesday, however, that they were developing a simple maternal blood test that can detect Down syndrome with a high rate of accuracy and greatly reduce the number of cases requiring an invasive test. Down syndrome occurs in about 1 in 800 births.

The new DNA sequencing technology relies on finding small bits of fetal DNA in the mother's blood, something that has been difficult to do until now. In the study, 753 pregnant women at high risk for having a baby with Down syndrome were tested.

Using the new technology, researchers found they could rule out Down syndrome in 98% of cases, sparing the women from further testing. The blood test did not produce false negative results (showing the fetus did not have the disorder when, in fact, the condition was present).

Currently, women in the United States are offered a blood test combined with an ultrasound test -- called the combined test -- to assess the risk of Down syndrome in pregnancy. The combined test still leaves an estimated 3% to 5% of women in need of invasive testing. The new blood test, which is conducted in the first trimester, could be used after the combined test or even as a first-tier test, said the study authors, from the Institute of Health Sciences in Hong Kong.